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The sol-gel technique Print E-mail
Oct 24, 2007 at 11:30 AM

Conventional glass preparation requires melting of the precursors at high temperatures, rapid cooling and subsequent vitrification of the glassy material. This procedure highly restricts choice of substances, which can be entrapped in the glass products. Basically only metal oxides and some inorganic salts can survive such drastic conditions avoiding thermal decomposition. Furthermore, the way in which conventional glass is produced makes thin films preparation extremely cumbersome and the only method of preparation of porous classical glasses requires etching or partial dissolving (e.g. Vycorâ glass). On the other hand glass and glassy materials possess several useful features for optical applications such as transparency, homogeneity, mechanical sturdiness, high refractive index etc.

An alternative approach to glass and glass-like materials is offered by the, so called, sol-gel technology. The process itself is known for more than a century, but it has gained a new importance in the last two decades after pioneering results of Dislich. He and other researchers improved the chemistry of the process so much that it is now possible to obtain samples in days (or even hours - in case of thin films) rather than months (or years) like in the case of the early samples.

The sol-gel technique is based on hydrolysis of liquid precursors and formation of colloidal sols. The precursors are usually organosilicates (e.g. TEOS - tetraethoxysilane) yielding silicate sol-gel materials. However, the method is not restricted to the silicon compounds - for example compounds of zirconium, vanadium etc. can be used as precursors leading to materials possessing different physico-chemical properties. Furthermore, it is possible to obtain modified organosilicate precursors with direct Si-C bonds (which do not undergo hydrolysis) and possessing terminal functional groups (e.g. -NH2, -SH2 etc.). Such precursors, either pure or mixed with the conventional ones, yield inorganic-organic materials with mechanical (e.g. elasticity) and physico-chemical properties (e.g. wetability) modified by the organic components of the inorganic polymer network. The functional groups can be also used for covalent binding of various chemicals (including biomolecules) giving specifically modified glassy materials.

In the case of the most often employed silicate sol-gel matrices, manufactured from hydrolizates of various alkoksysilanes, the chemical reactions involved in the gel formation involve the precursor hydrolysis:

Si - (OC2H5)4   +   4H2O   ®   Si(OH)4   +   4 C2H5OH

and the subsequent formation of the silicate network:

ºSi-OH   +   OH-Siº   ®   ºSi-O-Siº   +   H2O

or

ºSi-OC2H5   +   OH-Siº   ®   ºSi-O-Siº   +   C2H5OH

At this stage a wet gel is produced which, upon drying, yields porous xerogels. The drying is accompanied by liquid expulsion from the pores (syneresis) and substantial matrix shrinkage often leading to cracks (mainly due to the capillary pressure).

The hydrolysis process is significantly improved (accelerated) if conducted at pH ≠ 7. Thus, addition of acid (typically HCl(aq)) or base (typically NH4OH) speeds up this process. After the hydrolysis the acidity of the sol is neutralized slowly to approx. 7 pH, what stimulates the gelation process. At this stage a mechanically unstable "wet" gel is formed. Drying of wet gels (even at ambient temperatures) leads to xerogels ("dry gels"). Xerogels are stable, transparent and insoluble in water and most of organic solvents and porous solid materials.

In cases when fully-densified sol-gel glasses are sought, extensive drying at temperatures close to the vitrification temperatures will yield such materials (e.g. for silicate glasses it is necessary to heat at quartz vitrification temperature Vg ≈1300°C). This enables obtaining glasses in situations when it is not feasible via the conventional melting techniques (e.g. heavily-doped with certain temperature-resistant materials).

Since the early steps of the sol-gel process occur in liquid phase, it is possible to add basically any substance (as solutions or suspensions) at this stage. Simple mixing provides uniform distribution of the dopant within the liquid host phase. After the gelation the guest molecules become physically entrapped within the now solid host matrix. Furthermore, the hydrolysis, doping and gelation occur usually at ambient temperatures - allowing entrapment of even such delicate molecules as proteins without their decomposition. Sol-gel doped matrices, obtained in the above described manner, are of the form of xerogels and possess a network of internal pores and cavities enabling the entrapped molecules to interact with the surrounding medium. Furthermore, the doped matrices usually possess good optical characteristics (transparency and high refractive indexes). Those features are of key importance for production of optical sensors (optodes)

Thin sol-gel film on the fibre:

 

Bulk samples prepared by the sol-gel method:

 

Titania nanowires prepared by the hydrothermal sol-gel method:

     

Submicron silica spheres prepared by the sol-gel method:

kulki

 

   

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Last Updated ( Oct 24, 2007 at 02:37 PM )
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Oct 24, 2007 at 11:21 AM
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Last Updated ( Oct 24, 2007 at 02:37 PM )

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